This invention relates to a method for decreasing the catalytic activity of metal contaminants on cracking catalysts and for decreasing the hydrogen and coke formation on cracking catalysts. More specifically, this invention is directed to a method for reducing the coke and hydrogen formation caused by metal contaminants, such as nickel, vanadium and/or iron, which have become deposited upon cracking catalysts from feedstock containing same.
In the catalytic cracking of hydrocarbon feedstocks, particularly heavy feedstocks, vanadium, nickel and/or iron present in the feedstock becomes deposited on the cracking catalyst promoting excessive hydrogen and coke makes. These metal contaminants are not removed during conventional catalyst regeneration operations during which coke deposits on the catalyst are converted to CO and CO.sub.2. As used hereinafter the term "passivation" is defined as a method for decreasing the detrimental catalytic effects of metal contaminants such as nickel, vanadium and iron which become deposited on catalyst.
U.S. Pat. Nos. 3,711,422; 4,025,545; 4,031,002; 4,111,845; 4,141,858; 4,148,712; 4,148,714 and 4,166,806 all are directed to the contacting of the cracking catalyst with antimony compounds to passivate the catalytic activity of the iron, nickel and vanadium contaminants deposited on the catalyst. However, antimony compounds, alone, may not passivate the metal contaminants to sufficiently low levels particularly where the metal contaminant concentration on the catalyst is relatively high. U.S. Pat. No. 4,176,084 is directed to the passivation of metals contaminated catalyst in a regeneration zone operated for incomplete combustion of the coke to CO.sub.2 by periodically increasing the oxygen concentration above that required for complete combustion of the coke and by maintaining the temperature above 1300.degree. F. This patent does not disclose a method for passivating metals-contaminated catalyst in a system where the regeneration zone is routinely operated for complete combustion of the coke.
U.S. Pat. No. 2,575,258 is directed at passing catalyst which had been subjected to an oxidizing atmosphere in the regeneration step through a reducing atmosphere in the range of 850.degree.-1050.degree. F. to convert Fe.sub.2 O.sub.3 present with the catalyst to Fe.sub.3 O.sub.4.
U.S. Pat. No. 4,162,213 is directed at decreasing the catalytic activity of metal contaminants present in cracking catalyst by regenerating the catalyst at temperatures of 1300.degree.-1400.degree. F. in such a manner as to leave less than 0.10 wt. % residual carbon on the catalyst.
Cimbalo, Foster and Wachtel in an article entitled "Deposited Metals Poison FCC Catalyst" published at pp 112-122 of the May 15, 1972 issue of Oil and Gas Journal disclose that the catalytic activity of metal contaminants decrease with repeated oxidation and reduction cycles.
U.S. Pat. No. 3,718,553 is directed at the use of a cracking catalyst impregnated with 100-1000 parts per million by weight (WPPM) of iron, nickel or vanadium or a combination of these metals to increase the octane number of the cracked hydrocarbon products. This reference does not recognize that use of certain of these metals may adversely affect the catalyst selectivity or activity.
U.S. Pat. Nos. 3,479,279 and 4,035,285 disclose hydrotreating of catalytic cracker product cuts and recirculating this product to the catalytic cracker. Related U.S. Pat. Nos. 3,413,212 and 3,533,936 disclose the use of hydrogen donor materials for decreasing the rate of coke formation on cracking catalyst. These patents each disclose in Table V that hydrotreating a fraction from a catalytic cracking zone and returning the hydrotreated material with the cat cracker feed decreases the coke make in the catalytic cracking zone. These patents also disclose that the hydrotreated material preferably is a hydrogen donor material which releases hydrogen to unsaturated olefinic hydrocarbons in a cracking zone without dehydrogenative action. Suitable materials disclosed are hydroaromatic, naphthene aromatic and naphthenic compounds. Preferred materials are compounds having at least one and preferably 2, 3 or 4 aromatic nuclei, partially hydrogenated and containing olefinic bonds. The hydrogen donor material was hydrogenated by contacting the donor material with hydrogen over a suitable hydrogenation catalyst at hydrogenation conditions.
The subject invention is directed at a method for passivating metals contaminated cracking catalyst bypassing cracking catalyst from the reaction zone through a regeneration zone maintained under net reducing conditions and through a reduction zone maintained at an elevated temperature.